Switch mode power supplies are well known. They typically comprise or include an integrated circuit controller, inductors, capacitors and high current-carrying semi-conductors, typically diodes and field effective transistors (FET), which are controlled by the integrated circuit controller to switch and dissipate large amounts of power. With the exception of so-called smart MOSFETs, the diodes and FETs commonly used with switch mode power supplies do not have thermal protection and are susceptible to thermal damage.
A well-known feature of switch mode power supplies is their ability to provide a relatively constant output voltage, even when an input voltage to the supply is low or falling. In “automotive” applications, switch mode power supplies are often required to operate when a vehicle's storage battery terminal voltage is very low, e.g., about three volts, as often happens during engine cranking. When the terminal voltage of a storage battery voltage in a vehicle goes low, a switch mode power supply must then be able to carry correspondingly large amounts of input current in order to maintain its output voltage. Stated another way, in order to maintain a constant output power, the average input current to a switch mode power supply may have to increase substantially, if only for short periods of time.
It is also well known that switch mode power supply efficiency can decrease as the input voltage to the supply decreases. Efficiency losses further increase the average input current required to provide a constant output voltage.
Excessive power dissipation caused by excessive current flow through a semi-conductors of a switch mode power supply will cause the device to fail prematurely. If such devices do not have their own thermal protection, they will eventually be destroyed.
Prior art switch mode power supplies employ three different techniques to avoid destroying a semi-conductor by excessive heat dissipation caused by current flow. One solution is to over-size the external power components or over-design the heat sync for the devices such that the semi-conductors can operate indefinitely under worst case conditions. An obvious drawback of such a solution is the increased size and cost of external power components beyond what is necessary and is often an unacceptable solution where cost and physical size of an electronic circuit is important.
Another prior art solution to protecting devices from over temperature is to provide semi-conductors with their own built-in thermal protection. Many semi-conductor manufacturers offer so-called “smart MOSFETs” with built-in thermal shutdown mechanisms. These components are more expensive than unprotected MOSFETs and for that reason, using them is often undesirable.
A third method of protecting semi-conductors from over temperature destruction is to limit the time that they operate at an excessive input current in order to limit their temperature rise. Operating parameters of the power supply are used to determine the power dissipation on the external power components above typical values. A timer is started that determines the amount of time that the external components can be used before the power supply is shut off. When the timer reaches its predetermined maximum count, power to the semi-conductors is shut off, preventing them from destruction.
With regard to the third solution, a method and apparatus for optimizing or maximizing the time during which semi-conductors can be operated without destruction would be an improvement over the prior art.